What joist size to use for low-level deck?

[Zog]

Hi everyone. I have assembled the framing of the modular decking for my low level deck (see photos). What is in the photo is essentially a walkway, the main deck will be under the pergola and slightly higher. I've gained some confidence from what I've done so far and am considering using tuffblocks and building a framework rather than using the modular decking for the main deck. My question is just about the joist size. I'm not going to use a bearer and joists system, just a framework of joists, mainly because the Pergola is already there (before I bought the house) so I need to preserve the ceiling height of the pergola as much as possible.

I'm unsure about joist size and span of tuffblocks. The Tuffblock website has an article where they're using 2"x6" (45x150) joists, I've seen joists of 45x90 used in Australian based articles and the joists of the Good Times modular panels from Bunnings are 45x70. I'll probably use 140x19 merbau decking.

The pavers will be staying, they're not perfectly level, a small part of the deck will be on soil (the far sides in the second photo).

The total size of the main deck will be about 6.4m x 7.4m or 47.36 m2. It's a lot bigger than any articles I've seen about building a  low level deck. Any advice will be much appreciated.

[EricL]

Hello @Zog 

Thanks for sharing your question about your decking project. Just a friendly reminder to check your local council's rules and regulations regarding building a low-level deck in your area. I only mention it because of the proposed size of your deck. There might be size restriction rules that must be followed.

I suggest having a quick look at these guides - How to build a low-level deck by @Adam_W and How to build a deck by @MitchellMc. These guides will give you an idea of what to take into consideration when you draw your decking frame plan. I also suggest having a quick look at this discussion - Low-level composite deck over concrete and soil by @oninpena. It's an excellent example of a single level decking frame using 90x45 with mixed use footing.

I propose looking at the Builders Edge Pedestal Fee, simply screw the adjustable foot to the wood substructure, then adjust the height by screwing the adjusting unit. The bearing plate with the bracket can be freely rotated. 

Here is a link for ideas and inspiration: Top 10 most popular deck projects

I also suggest having a look at the Bunnings deck building guide: 

• How to plan out your deck
   
• How to set out a deck
   
• How to install stumps
   
• How to install bearers
   
• How to install joists
   
• How to lay decking
   
• How to add a step to your deck.

I suggest doing a basic frame layout on paper to give you an idea of how the decking frame will be laid out and in which direction the decking panels will be facing. While you're drawing your plans, it would be a good opportunity to think about whether you are installing a picture frame and breaker boards for your decking panels.   

If you have any other questions we can help with, please let us know.

Eric

[Zog]

Thanks,

I'm not including a breaker board as, of the many decks I've seen at friends houses over the years, none of them had breaker boards, and they didn't buckle. I'm going to use facia boards rather than picture frame. Thanks for the  builders edge pedestal feet. As the surface I'm putting the deck over is not perfectly flat, they will be more suitable than tuffblocks.

I had already seen some of those videos and others and was just confused about the joist wood because of the different sizes people have used. I think I'll go with 45x140. That way I'll be able to use wider spacing of the footings which will make the levelling easier.

[JacobZ]

Hi @Zog,

There's no harm in not using a breaker board. They are commonly used with composite decking products as they expand and contract quite a lot with the heat. They can also be used if you have particularly long runs and don't want butt joins in your decking.

A fascia board will look fantastic and I'm excited to see how it turns out.

140x45 is a great size for decking that is able to span 2.6m between vertical support. I use the Hyne DeckJoist Span Table when planning decks if you wanted to have a quick read.

Keep us in the loop as your deck project continues, it's going to be brilliant once it's done.

Jacob

[Zog]

Thanks Jacob and Eric.

Thanks for the deck joist span table. 

As the deck is 6.4x7.4m, the framing joists will be 1.4m too short and the joists themselves .4 m too short (assuming the length of the 45x140 is 6m).

I expect that I would place a support directly under where they're joined. What kind of join is necessary ? 

And is it best to add .4m and 1.4m sections to the ends or should I join two larger sections so the joins are in the middle  ?

[JacobZ]

Hi @Zog,

In this situation, I would suggest joining the timber members directly over a support using a half lap join. See the below render as an example of this.

Using a 140x45mm joist, I would suggest cutting a 70x70mm square from each of the timbers and using a ZENITH 14G - 8 x 125mm Class 3 Galvanised Bugle Batten Screw straight down through the join to lock them together. 

As you have said, these joins should be located directly over your vertical supports. I would however suggest that they should not sit on the corresponding support with the joist next to it. If you have for example 4 supports per joist, this join should never be directly next to the join in the one next to it. If they line up with the join next to it, you are creating a line of weakness throughout your deck. Randomness in terms of join locations will create strength.

Let me know what you think and if you've got further questions, please don't hesitate to ask.

Jacob

[Zog]

Hi Jacob, yep make sense. I'll let you know how I go.

[Zog]

Hi, @JacobZ 

is there a recommended height for a pergola ceiling? I'm just wondering how much room I have to play with for the deck height. The pergola is hexagonal and the ceiling slopes downward from the centre, so at the lowest height the celing is 2.47 metres, in the centre it's 3.12 mtrs.

[JacobZ]

Hi @Zog,

Just as an example, the minimum allowable ceiling height in a home is 2.4m for it to be considered up to code. I'm not aware of a rule against a pergola being shorter than this, but anything below about 2.1m in height would start to feel extremely cramped. 

I would be aiming for the ceiling height to be no lower than 2.2m in all areas. 

Would this be doable with the deck you are looking at?

Let me know if you have any further questions.

Jacob

[Zog]

If I use 45x140 joists with Builders Edge pedestals set at 90, plus decking board thickness, 140+90+19=249. It will bring the outside, the lowest part of the ceiling to about 2.25 mtrs. That's not counting the fascia around the pergola which will then be about 2.05 mtrs.

I could change it to 45x90 joists which would bring the fascia to 2.1 mtrs (and the ceiling to 2.3) and tuff blocks appear to be lower (but I can't find a measurement).